1. Field of the Invention
The present invention relates to a communication apparatus, in particular to a spread-spectrum communication apparatus for use in mobile communications, wireless LAN, or the like.
2. Description of the Related Art
Spread-spectrum techniques expand bandwidth to gain transmission advantages. At a transmitter, pseudorandom spreading distributes the transmitter's power over a much wider frequency range, with much lower power density. Because the spreading is reversed at a receiver, narrow-band interfering signals are spread before demodulation, and wide-band interfering signals remain wide-band. The interference power density in the reconstructed narrow band remains low, while the higher power density of the desired signal is available to the receiver demodulator. Therefore, interference is reduced.
However, as the strength of the interfering signal becomes greater, the S/N ratio will decrease even if a spread spectrum technique is used. In such a case, it is necessary to reduce the power of the interfering signals in advance of despreading.
It may be possible to selectively eliminate the interference by using a band elimination filter (BEF). A programmable band-elimination filter such as an adaptive interference suppressor is often used.
In such case, it is preferable to make the band-width of the filter narrower in comparison with the distributed frequency range to avoid excess elimination of the desired spread signal.
However, as the band-width of the filter decreases, the scanning speed in the process of detecting interference decreases. This reduces the number of interfering signals which can be detected in a unit time.
In a conventional preprocessing technique, interfering signals are first detected utilizing a bandpass filter. Then, the center frequency of the interfering signal is identified. Finally, the interfering signal is eliminated utilizing a band-elimination filter. The foregoing process must be performed for each frequency window being examined.
It is preferable to make the width of the frequency window as narrow as possible because the narrower the window, the smaller the amount of object signal which will be eliminated. Usually, the foregoing steps are carried out by using a digital signal processing technique and the window must scan over a wide frequency range. As a result, when using a narrow window, it will take more time to detect, identify and eliminate the interfering signal. This significantly slows down the detection process.
On the other hand, it may be possible to detect several interference signals in the frequency region in parallel by means of plural band-elimination filters. But, such a method will require a complex signal processing circuit.
In another method, a suppression filter having saturation characteristics in a frequency region is utilized to suppress signals whose power exceeds a predetermined level which is higher than the power level of the desired spread signal. Thereby, the total power of the interference is decreased relative to that of the desired signal.
It is known that a device such as a diode or an amplifier has a saturating characteristic. However, when using such devices, the power of the desired signal is also decreased because such devices have saturating characteristics in the entire frequency region. In other words, the power of the desired signal is also decreased in a frequency region in which no interference appears.
Gevargiz, Das, and Milstein developed techniques for adaptive narrow-band interference rejection in a DS spread-spectrum intercept receiver using transform domain signal processing techniques (IEEE Transactions on Communications, Vol. 37, No.12, December 1989). In that receiver, a chirp filter is used as a suppressor. However, the structure of the chirp filter is relatively complex.